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Chapter 5—Gases

5.1: Substances That Exist as Gases

Air: 78% N2, 21% O2, 1% Other (such as CO2)

Ionic compounds do not exist as gases at 25 degrees Celsius as well as 1 atm (due to strong electrostatic forces holding cations/anions together in an ionic solid).

The stronger the attractions (intermolecular forces), the less likely the compound can exist as a gas at ordinary temperatures.

The characteristics of gases include: assuming the volume/shape of their containers, most compressible of the states of matter, will mix evenly/completely when confined in the same container, lower densities than liquids and solids.

5.2: Pressure of A Gas

Gas molecules are constantly in ...view middle of the document...

• A barometer is an instrument for measuring atmospheric pressure (long glass tube, closed at one end/filled with mercury). On the other hand, a manometer is a device used to measure the pressure of gases other than the atmosphere (closed-tube measures pressures below atmospheric pressure and open-tube is suited for pressures equal to or greater the atmospheric pressure).

• Standard atmospheric pressure (1 atm) is equal to the pressure that supports a column of mercury exactly 760 mm or 76 cm high at 0 degrees Celsius at sea level.

• 1 atm = 760 mmHg (mmHg = pressure exerted by a column of mercury 1 mm high; mmHg is also known as torr)

• 1 atm = 101,325 Pa or 1.01325 x 105 Pa or 1.01325 x 102 kPa

Example 5.1: The pressure outside a jet plane flying at high altitude falls considerably below standard atmospheric pressure. Therefore, the air inside the cabin must be pressurized to protect the passengers. What is the pressure in atmospheres in the cabin if the barometer reading is 688 mmHg?

Example 5.2: The atmospheric pressure in San Francisco on a certain day was 732 mmHg. What was the pressure in kPa?

5.3: The Gas Laws

The Pressure-Volume Relationship: Boyle’s Law

• Boyle’s law states: the pressure of a fixed amount of gas at a constant temperature is inversely proportional to the volume of the gas.

• Mathematical Equation: P 1/V

• P = k1 x 1/V where k1 is a proportionality constant that equals to nRT (rearranges to PV = k1)

• P1V1 = k1 = P2V2 or P1V1 = P2V2 where V1 and V2 are volumes at P1 and P2 respectively

The Temperature-Volume Relationship: Charles’s and Gay-Lussac’s Law

• Absolute zero is theoretically the lowest attainable temperature, and Kelvin in 1848 defined this as -273.15 degrees Celsius. The absolute temperature scale was then set up (now known as Kelvin temperature scale with absolute zero as the starting point)

• One kelvin is equal in magnitude to one degree Celsius on the Kelvin scale.

• The dependence of the volume of gas on temperature is: V T, or V = k2T or V/t = k2 (where k2 is the proportionality constant, and it is equal to nR/P).

• Charles’s and Gay-Lussac’s law states: the volume of a fixed amount of gas maintained at constant pressure is directly proportional to the absolute temperature of the gas

• The volume-temperature relationships can be defined as: where V1/T1 = k2 = V2/T2 or V1/T1 = V2/T2 where V1 and V2 are volumes of the gases at temperatures T1 and T2 (in Kelvins) respectively

• The pressure of a gas is proportional to temperature: P T, or P = k3T, or P/T = k3

k3 = nR/V, so P1/T1 = k3 = P2/T2 or P1/T1 = P2/T2 where P1 and P2 are pressures of the gases at temperatures T1 and T2 respectively

The Volume-Amount Relationship: Avogadro’s Law

• Avogadro’s law states: at constant pressure and temperature, the volume of a gas is directly proportional to the number of moles of the gas present.

• Mathematical expression: V n, or V = k4n (n is the...

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